|MMAP(2)||System Calls Manual||MMAP(2)|
mmap — map files
or devices into memory
*addr, size_t len,
function causes the contents of fd, starting at
offset, to be mapped in memory at the given
addr. The mapping will extend at least
len bytes, subject to page alignment restrictions.
The addr argument describes the address
where the system should place the mapping. If the
MAP_FIXED flag is specified, the allocation will
happen at the specified address, replacing any previously established
mappings in its range. Otherwise, the mapping will be placed at the
available spot at addr; failing that it will be placed
"close by". If addr is
NULL the system can pick any address. Except for
MAP_FIXED mappings, the system will never replace
The len argument describes the
minimum amount of bytes the mapping will span. Since
pages into memory, len may be rounded up to hit a page
boundary. If len is 0, the mapping will fail with
If an fd and offset are specified, the resulting address may end up not on a page boundary, in order to align the page offset in the addr to the page offset in offset.
The protections (region accessibility) are specified in the
prot argument. It should either be
PROT_NONE (no permissions) or the bitwise OR of one
or more of the following values:
The flags parameter specifies the type of the mapped object, mapping options, and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references. Sharing, mapping type, and options are specified in the flags argument by OR'ing the following values. Exactly one of the first two values must be specified:
Any combination of the following flags may additionally be used:
MAP_ANONmust currently be -1 indicating no name is associated with the region.
Finally, the following flags are also provided for source compatibility with code written for other operating systems, but are not recommended for use in new OpenBSD code:
MAP_PRIVATE, modifications made by others are not visible. On OpenBSD this behaves just like
mmap() function returns a pointer to
the mapped region if successful; otherwise the value
MAP_FAILED is returned and the global variable
errno is set to indicate the error. A successful
mmap() will never return the value
mmap() will fail if:
PROT_READwas specified as part of the prot parameter and fd was not open for reading. The flags
PROT_WRITEwere specified as part of the flags and prot parameters and fd was not open for writing.
MAP_SHAREDwere both specified.
MAP_FIXEDwas specified and the addr parameter was not page aligned.
MAP_FIXEDwas specified and the addr parameter wasn't available.
MAP_ANONwas specified and insufficient memory was available.
PROT_EXECmappings are not permitted unless the filesystem is mounted
wxallowedand the process is link-time tagged with
wxneeded. (See also
kern.wxabortin sysctl(2) for a method to diagnose failure).
mmap() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
A fully functional
mmap() system call
first appeared in SunOS 4.0 and has been available since
IEEE Std 1003.1-2008
(“POSIX.1”) specifies that references to pages beyond
the end of a mapped object shall generate a
signal; however, on some architectures OpenBSD
SIGSEGV signal in this case instead.
Additionally, IEEE Std 1003.1-2008
(“POSIX.1”) specifies that
mmap() shall fail with
EINVAL if neither
is specified by flags; however, for compatibility with
old programs, OpenBSD instead defaults to
MAP_SHARED for mappings of character special files,
MAP_PRIVATE for all other mappings. New
programs should not rely on this behavior.
Due to a limitation of the current vm system (see
uvm_init(9)), mapping descriptors
PROT_WRITE without also specifying
PROT_READ is useless (results in a segmentation
fault when first accessing the mapping). This means that such descriptors
must be opened with
O_RDWR, which requires both read
and write permissions on the underlying object.
|March 2, 2021||OpenBSD-current|